As noted by A. Bannister and S. Raymond in Surveying, Pitman Publishing Ltd., London, 1977, the general notion of surveying was known and practiced more than 2000 years ago. The methods used at that time were simple but subject to consistency errors and required considerable time to perform. Surveying instruments have improved considerably since about 1900, taking advantage of advances in electronics, optics and other related disciplines. Recently, lasers, electro-optics, wave interaction and phase detection have been introduced into, and used in, surveying activities.
Use of a laser beam projector for surveying operations is disclosed in U.S. Pat. No. 3,471,234, issued to Studebaker. The beam rotates over terrain to be surveyed, and a beam point may be directed to a particular location and used to measure elevation and angular displacements within the region covered by the rotating beam.
Altman, in U.S. Pat. No. 3,669,548, discloses a method for determining a ship""s heading or bearing, using an electro-optical angle measuring device that determines angles relative to a horizontal datum line. A plurality of parallel light beams, spaced apart by known, uniform distances and oriented at a known angle, forms a one-dimensional grid that covers the region where the ship is located. A rotating reflecting telescope on the ship has its axis aligned with one of the parallel light beams. The angle of the ship""s longitudinal axis relative to the known direction of the parallel light beams is then easily read off to determine the ship""s heading. This approach would not be suitable where the ship or other body whose angular orientation is to be determined can move over a large region.
Remote measurement of rotation angle of an object of interest by use of polarized light and electro-optical sensors is disclosed by Weiss et al in U.S. Pat. No. 3,877,816. The intensity of light transmitted serially through two linear polarization filters is proportional to the square of the cosine of the angle between the two polarization directions, and the proportionality constant can be determined by experiment. Unpolarized light transmitted along a first reference path with fixed polarization directions is compared with unpolarized light transmitted along a second, spatially separated and optically baffled path in which the polarization direction of one polarizer may vary. One or two light polarizers in each light beam path rotates at a constant angular velocity, which is the same for each path, and the difference in phase of the two received light signals is a measure of the angle of rotation of a polarizer (or the body to which the polarizer is attached) in the first path and a polarizer in the second path.
An optical-electronic surveying system that also determines and displays the angular orientation of a survey pole relative to a local horizontal plane is disclosed in U.S. Pat. No. 4,146,927, issued to Erickson et al. The system can receive and process range measurements directly from an electronic distance meter located near the system.
U.S. Pat. No. 4,443,103, issued to Erdmann et al, discloses use of a retro-reflective, electro-optical angle measuring system, to provide angle measurements after interruption of a signal that initially provided such information. A light beam is split into two beams, which intersect on a scanning mirror, which rotates or vibrates about a fixed axis, and the two beams are received at different locations on a retro-reflective tape positioned on a flat target surface on the target whose rotation is to be measured. These two beams form a plane that moves as the scanning mirror moves, with a reference plane being defined by the mirror at rest in a selected position. The scanning mirror sweeps the plane of the two beams across the target surface. A rotation angle of the target surface relative to the reference plane is determined, based upon the time difference between receipt of light from each of the two retro-reflected beams. The beam interception times coincide only if an edge of the retro-reflective tape is parallel to the reference plane. If receipt of light from the two retro-reflected beams is displayed on a synchronized, two-trace oscilloscope screen, the two xe2x80x9cblipsxe2x80x9d corresponding to receipt of these two beams will have a visually distinguishable and measurable time difference xcex94t, as indicated in FIGS. 2A, 2B and 2C of the Erdmann et al patent. The time difference xcex94t will vary as the scanning mirror moves. A second Erdmann et al U.S. Pat. No. 4,492,465, discloses a similar approach but with different claims.
xe2x80x9cTotal stationxe2x80x9d electronic instrumentation for surveying, and more particularly for measurement of elevation differences, is disclosed by Wells et al in U.S. Pat. No. 4,717,251. A rotatable wedge is positioned along a surveying transit line-of-sight, which is arranged to be parallel to a local horizontal plane. As the wedge is rotated, the line-of-sight is increasingly diverted until the line-of-sight passes through a target. The angular displacement is then determined by electro-optical encoder means, and the elevation difference is determined from the distance to the target and the angular displacement. This device can be used to align a line-of-sight from one survey transit with another survey transit or to a retro-reflector. However, the angular displacement is limited to a small angular sweep, such as 12xc2x0.
U.S. Pat. Nos. 4,667,203, 5,014,066 and 5,194,871, issued to Counselman, disclose methods for measuring the baseline or separation vector between two survey marks, using GPS carrier phase signals. These methods use radiowave interferometric analysis of carrier phase signals received from many GPS satellites. This often requires observation time intervals of substantial length (≈5000 seconds) for the baseline vector determination, with reported inaccuracies less than 5 cm.
In U.S. Pat. Nos. 4,924,448 and 5,231,609, Gaer discloses a method for using GPS signals for ocean bottom mapping and surveying. Two ships, each with a GPS station (GPS antenna and receiver/processor) travel parallel routes a fixed distance apart. The first ship transmits a sonic signal toward a location on the ocean bottom, and the specularly reflected portion of this signal is received and analyzed to determine the location of the portion of the ocean bottom that reflected the signal.
Fodale et al disclose an electro-optical spin measurement system for use in a scale model airplane wind tunnel in U.S. Pat. No. 4,932,777. Optical targets (six) to receive and sense one or several light beams are located under the fuselage at the nose tip, on each of two sides of the fuselage, and under each wing tip, and a plurality of optical receivers are positioned on the perimeter of the wind tunnel to receive light from the optical targets at various angles, to determine airplane angle of attack and roll angle. The time-synchronized signals received at each receiver are recorded for subsequent analysis.
In U.S. Pat. No. 4,954,833, issued to Evans et al, information on deflection of the local vertical (obtained from gravity measurements) is combined with geodetic azimuth estimated from GPS signals to obtain an astronomical azimuth. This azimuth can be used for ballistic projectile delivery to a selected target. This method does not focus on integration of GPS operation with theodolite operation but, rather, seeks to avoid use of a theodolite to obtain the astronomical azimuth.
Kroupa et al, in U.S. Pat. No. 4,988,189, disclose use of a passive rangefinding system in combination with an electro-optical system, using image information obtained at two or more electro-optical system positions. A method for simultaneously measuring the difference between orthometric (geoidal) height and height above a given ellipsoid for a site on the Earth""s surface is disclosed by Evans in U.S. Pat. No. 5,030,957. Two or more leveling rods are held at fixed, spaced apart locations, with a known baseline vector between the rods. Each levelling rod holds a GPS signal antenna, receiver and processor that determines a GPS location for each rod. The geometric height of the GPS antenna (or of the intersection of the rod with the Earth""s surface) is determined for each rod, and the geometric height difference is determined, using standard GPS survey measurements (accurate to within a few centimeters). A comparison of the orthometric height, usually found using a spirit level, and the height above the ellipsoid, obtained from a GPS measurement, provides a measure of the local gravitational field. The patent does not indicate, or perhaps recognize, advantages of use of height information to aid the GPS carrier phase initialization process but treats the GPS and the levelling rods as separate, non-interacting systems.
Ohishi et al disclose an optical distance measuring instrument using light transmitted and returned by retro-reflection in U.S. Pat. No. 5,054,911. A light beam pulse generated at the instrument is split into two pulses; one pulse is immediately received by a laser diode as a reference pulse. The other pulse is transmitted to a retro-reflector at a remote or adjacent target and returned to the instrument by retro-reflection thereat. The returning pulse is received by an optical fiber, having a known time delay xcex94t and then received by the laser diode to provide a second pulse. The time delay xcex94t is subtracted from the difference of arrival times of the two pulses and divided by 2c (c=ambient medium light velocity) to obtain the distance from instrument to target.
A somewhat unclear disclosure of a beam alignment apparatus and method is presented in U.S. Pat. No. 5,060,304, issued to Solinsky. Two substantially identical beam acquisition apparati are spaced apart from each other, each apparatus including two identical parabolic mirrors with parallel axes, each mirror having an axial aperture through which an electromagnetic wave beam passes and having a second smaller mirror located at the parabola""s focal point. Each parabolic mirror has a third mirror consisting of a plurality of small retro-reflectors, located adjacent to but behind the parabolic mirror so that the parabolic mirror lies between the second and third mirrors. One parabolic mirror in each pair receives light from a transmitter positioned behind the mirror aperture and transmits this beam in a direction parallel to the mirror axis. The other parabolic mirror in each pair receives an incident beam propagating parallel to its axis and reflects this light to a receiver located behind the mirror aperture. One of the parabolic mirror pairs is operated in a search mode (moving) at a first selected frequency f1. The second parabolic mirror pair is operated in a xe2x80x9cstarexe2x80x9d mode at a selected frequency f2xe2x89xa0f1. As the two mirror pairs come close to alignment with each other, the mirror pairs sense this by receipt of a retro-reflected beam or a directly transmitted beam, the distinction being made by the frequency of the beam received. The search mode mirror pair, and then the stare mode mirror pair, can then be brought into alignment with each other.
A surveying instrument that uses GPS measurements for determining location of a terrestrial site that is not necessarily within a line-of-sight of the surveyor is disclosed in U.S. Pat. Nos. 5,077,557 and 5,233,357 issued to Ingensand and to Ingensand et al. The instrument uses a GPS signal antenna, receiver and processor, combined with a conventional electro-optical or ultrasonic range finder and a local magnetic field vector sensor, at the surveyor""s location. The range finder is used to determine the distance to a selected mark that is provided with a signal reflector to return a signal issued by the range finder to the range finder. The magnetic field vector sensor is apparently used to help determine the surveyor""s location and to determine the angle of inclination from the surveyor""s location to the selected mark.
U.S. Pat. No. 5,101,356, issued to Timothy et al, discloses a moving vehicle attitude measuring system that mounts three GPS signal antennas, each attached to a GPS receiver/processor, in a non-collinear configuration on the vehicle at predetermined distances from each other. The phases of rf signals arriving at the antennas are compared to determine the angular orientation of the plane containing the three antennas, and the angular orientation of the vehicle that carries these antennas.
Method and apparatus for measuring the relative displacement of two objects, applicable to monitoring of movement of adjacent material along an earthquake fault, is disclosed in U.S. Pat. No. 5,112,130, issued to Isawa. First and second optical distance measuring instruments (ODMIs) are placed at known locations astride a selected line (e.g., a fault line). First and second optical reflectors, also astride the selected line, are spaced apart by known distances from the first and second ODMIs. Distances from the first ODMI to the second reflector and from the second ODMI to the first reflector are measured ab initio and compared with subsequent readings of these two distances. If one or both of these distances changes, the magnitudes of the changes are used to determine how far the Earth on one side of the line has moved relative to the Earth on the other side of the line, as might occur in a slip along a fault line.
U.S. Pat. No. 5,142,400, issued to Solinsky, discloses a method for line-of-sight acquisition of two optical beam transceivers suitable for use in satellite communications. A first beam transceiver has an optical retro-reflector and initially operates in a passive or xe2x80x9cstarexe2x80x9d mode, with its beam transmitted in a fixed direction. A second transceiver performs a search over 2xcfx80 steradians with its optical beam until it receives, from the first transceiver, either (1) a return of its own beam or (2) a distinguishable beam from the first transceiver. Boresight alignment is then maintained after beam-to-beam acquisition.
Ghaem et al disclose an electronic direction finder that avoids reliance on sensing of terrestrial magnetic fields for establishing a preferred direction for satellite signal acquisition in U.S. Pat. No. 5,146,231. The apparatus uses a receiver/processor for GPS or similar navigation signals received from a satellite, and requires (stored) knowledge of the present location of at least one reference satellite from which signals are received. The orientation of the finder or its housing relative to a line of sight vector from the finder to this reference satellite is determined. This orientation is visually displayed as a projection on a horizontal plane. Any other direction in this horizontal plane can then be determined with reference to this projection from a knowledge of the reference satellite location.
U.S. Pat. No. 5,146,290, issued to Hartrumpf, discloses apparatus for determining the position and angular orientation of an object. A partially silvered hemispherical light reflector is fixed to some part of the object, and two spaced apart laser beams are directed to intersect at the hemisphere center, to be (partly) retro-reflected at the hemisphere reflector surface, and to return toward the laser sources, to be detected by photodetectors located adjacent to each laser source. A portion of the beam from each laser source is transmitted through the hemispherical reflector and is received by a line or plane of photodetectors positioned on a plane behind the hemispherical reflector. As the object is translated or rotated, the locations where the reflected and transmitted beams are received by the photodetector arrays changes in a manner that can be related to the translation and/or rotation of the object.
In U.S. Pat. No. 5,155,490, issued to Spradley et al, a method using three fixed-location GPS base stations for geodetic surveying is disclosed. Long observation time intervals are used to determine receiver clock drift at each GPS station. A mobile GPS station then determines its location with enhanced accuracy using the network of these fixed base stations with known locations.
Ferguson et al disclose a method for surveying using a rotary wind aircraft in U.S. Pat. No. 5,182,566. An initial location of a GPS station mounted on the aircraft is initially determined with high accuracy. The GPS station is then moved to a desired survey point while maintaining lock on the signals being received from GPS satellites.
Method and apparatus for determination of the roll, pitch and azimuth orientation angles for a survey instrument by analyzing GPS carrier phase signals is disclosed by Knight in U.S. Pat. No. 5,296,861. A direct phase integer search to resolve the ambiguities for the carrier phase integers is performed, using a maximum likelihood to identify one or more acceptable solutions.
Blume discloses a survey or identification system to allow a space platform to distinguish between a friendly object and an unfriendly object in U.S. Pat. No. 5,382,957. The platform, which includes a GPS receiver/processor and antenna, transmits an encrypted interrogation signal to the object, which can be positioned tens or hundreds of kilometers from the platform, requesting certain information including the location coordinates of the object. Simultaneously, the platform uses high directivity radar and line of sight measurements to estimate the object range and the object location coordinates, using the platform""s GPS-based knowledge of its own location. A friendly object will reply to the encrypted interrogation signal with an authenticating reply, including the GPS-determined location coordinates of the object. A receiver on the platform receives the object location coordinates from the object and compares these coordinates with its own estimate of the object location coordinates. If the object-supplied object location is within a determinable distance of the platform-supplied object location and all other authenticating replies from the object are appropriate, the platform authenticates the object as a xe2x80x9cfriendlyxe2x80x9d object.
Method and apparatus for GPS-assisted airborne surveying, which uses differential GPS corrections and which compensates for the thermal inertia of the object being surveyed, is disclosed in U.S. Pat. No. 5,445,453, issued to Prelat. Two airborne surveys are conducted at spaced apart times to assess the effects of maximum and minimum radiation temperatures on the object being surveyed.
A theodolite and tape have traditionally been used to measure horizontal and vertical angles and distances in terrestrial surveying. More recently, digital theodolites, as described in U.S. Pat. No. 3,768,911, issued to Erickson, and electronic distance meters (EDMs), as described by Hines et al in U.S. Pat. No. 3,778,159, have supplanted the theodolite and tape approach. Combination of an optical angle encoder and an EDM in an integrated package (called an xe2x80x9celectronic total stationxe2x80x9d), as disclosed in U.S. Pat. No. 4,146,927, issued to Erickson et al, has led to automation of field procedures, plan production and design work.
Several limitations exist in use of a conventional total station. First, it is difficult to quickly establish the angular orientation and absolute location of a local survey or datum. Many surveys are not related to a uniform datum but exist only on a localized datum. In order to accurately orient a survey to a global reference, such as astronomical north, a star observation for azimuth is often used that requires long and complicated field procedures. Second, if a survey is to be connected to a national or state geodetic datum, the survey sometimes must be extended long distances, such as tens of kilometers, depending upon the proximity of the survey to geodetic control marks. Third, the electronic total station relies upon line-of-sight contact between the survey instrument and the rodman or pole carrier, which can be a problem in undulating terrains.
These systems do not provide the benefits of an integrated SATPS and terrestrial total station instrument. What is needed is a system that provides: (1) rapid azimuth and location determination in a fixed reference frame; (2) prompt resolution of the carrier phase ambiguities that occur in a SATPS; (3) distance and angle information without requiring line-of-sight contact between a reference station and a mobile station; (4) flexibility and fail-safe capability for cross-checking, and calibrating the respective error sources in, the location information provided by the SATPS and by the terrestrial positioning system; and (5) capability for accounting for height differences between the geoid and ellipsoid over the local survey area.
These needs are met by the invention, which provides a surveying system that combines Satellite Positioning System (SATPS) techniques with new and with known survey techniques. The apparatus includes a first or instrument survey station that provides a reference for the survey, a second or mobile survey station that is spaced apart from the first station and acts as a mobile measurement unit for the survey. Each of these stations receives SATPS signals and determines its location using these signals. More than one mobile station can be used simultaneously with one instrument station.
The instrument survey station includes a first Satellite Positioning System (SATPS) antenna and first SATPS receiver/processor, connected together, for receiving SATPS signals, representing pseudorange or carrier phase, from two or more satellites and for determining the instrument station location according to the SATPS signals. The instrument station also includes an instrument station communications antenna, connected to the first SATPS receiver/processor, for transmitting or receiving station location and satellite attribute information. The instrument station also includes an electronic distance meter (EDM) and digital theodolite, whose spatial orientation can be varied arbitrarily, connected to the first SATPS receiver/processor, for transmitting electromagnetic waves having a selected wavelength and for determining the distance from the instrument station to the mobile station by receipt of a return electromagnetic signal from the mobile station, for determining the elevation difference, if any, between the instrument station and the object, and for determining the angular displacement between a line drawn from the instrument station to the object and a selected reference line.
The mobile survey station includes a second SATPS antenna and second SATPS receiver/processor, connected together, for receiving SATPS signals, representing pseudorange or carrier phase, from two or more SATPS satellites and for determining the mobile station location according to the SATPS signals. A mobile station communications antenna, connected to the second SATPS receiver/processor, for communicating with the instrument station communications antenna and for transmitting to the instrument station a signal containing feature and attribute information and information on the location of the mobile station as determined by the SATPS satellite signals, is also included in the mobile station. The mobile station also includes an electronic distance meter responder, adapted to receive the electromagnetic waves transmitted by the electronic distance meter and to provide a return electromagnetic signal that is received by the electronic distance meter at the instrument station. The instrument station communication means and the mobile station communication means are connected by a data link for transferring information from one station to the other station.
In a first embodiment, the location of the mobile station is initially known with high accuracy and the second SATPS receiver/processor is adapted for subsequently determining the difference, if any, between the xe2x80x9cknownxe2x80x9d pseudorange or carrier phase signals the mobile station should receive at its known location, and the corresponding signals actually received at the mobile station. These corrections are used at the mobile station, or transmitted to the instrument station, to enhance the accuracy of the computed location of one or both of these stations.
In a second embodiment, a remote station whose location is known with high accuracy is added. The remote station includes a third SATPS antenna and third SATPS receiver/processor, connected together, for receiving SATPS signals, representing pseudorange or carrier phase, from two or more SATPS satellites and for determining the location of the remote station according to the SATPS signals. A remote station communications antenna, connected to the third SATPS receiver/processor, to communicate with the instrument station communication means and/or with the mobile station communication means and to transmit to at least one of these stations a signal containing location and attribute information, as determined by the SATPS satellite signals, is also included in the remote station. The location of the remote station is known with high accuracy. The SATPS signal receiver/processor at the remote station is adapted for determining the difference, if any, between the xe2x80x9cknownxe2x80x9d pseudorange or carrier phase signals the remote station should receive at its known location, and the pseudorange or carrier phase signals actually received at the remote station.
The invention provides a xe2x80x9ctotal SATPS stationxe2x80x9d, including first and second spaced apart SATPS stations whose relative separation is known with high accuracy, as an integrated supplement to survey equipment conventionally used. Each of the first and second SATPS stations includes an SATPS antenna and SATPS receiver/processor that receive signals from two or more SATPS satellites and process these signals to partly or fully determine the position of the receiving SATPS antenna. The first and second SATPS antenna and associated SATPS receiver/processor may be retrofitted within first and second housings, respectively, that contain conventional first and second electro-optical survey instruments, respectively, used to determine the bearing, length of, and/or height difference of a separation vector joining the two electro-optical survey instruments. Differential corrections are provided for the SATPS signals received at the instrument survey station and/or at the mobile survey station, to increase the accuracy of, or to correct, the location determined for that station.
The invention uses certain electro-optical survey measurements, implemented by use of one or more signal retro-reflectors that operate in the microwave, infrared, visible or ultraviolet wavelength ranges, to determine the bearing, length of, and/or height difference of a separation vector joining the first and second stations. This requires that the two stations have line-of-sight visual contact. The primary object is to implement carrier phase positioning (accurate to within a few centimeters), as opposed to the less accurate code phase positioning, using the SATPS satellite signals. Carrier phase positioning is implemented by causing two or more SATPS stations to track a common group of SATPS satellites. The measurements are then merged and either processed in real time, or post-processed, to obtain data useful in determination of the location of any stationary or mobile SATPS station near an SATPS instrument station. Real time positioning requires transfer of SATPS data between a instrument station and a mobile station, using a data link that need not rely upon line-of-sight communication.
One problem that must be overcome initially in use of carrier phase positioning is the presence of phase integer ambiguities in the carrier phase measurements for the tracked satellites. An integer search technique for identification of the phase integers often takes account of the statistical nature of discrete integer combinations that are realistic candidates for the proper phase integers. The number of possible combinations to be searched is enormous, unless the number of candidates can be reduced ab initio. If the relative location of two SATPS stations is known precisely, the number of initial phase integer combination candidates can be reduced to as few as one. If the horizontal or vertical separation distance between the two stations is known with high accuracy in the SATPS frame, the number of phase integer combination candidates can be reduced to a modest number that can be searched relatively quickly and can reliably produce the correct results. The number of phase integer combination candidates is reduced by sequentially applying position information provided by the electro-optical survey measurements
Another serious problem with carrier phase positioning is the possibility of SATPS signal interruptions at one or both SATPS stations. When a SATPS satellite signal is lost, the phase integer(s) must be redetermined. Signal interruption can easily occur in urban or other built-up areas where tall structures interfere with or produce multipath SATPS signals. A separation vector between two SATPS stations, specified by three coordinate differences, or by a vector magnitude and two or more spherical angles relative to a fixed direction, may be known initially. However, one or both of these stations may have moved when the signal is interrupted so that the separation vector must be established again.
The invention provides a separation vector, between the two stations by use of one or more wave retro-directors that are mounted on the second station and facing the first station. An electromagnetic wave beam (xe2x80x9clight beamxe2x80x9d) is directed from the first station toward the second station, and the beam is retro-reflected from the second station toward the first station. The station-to-station separation vector is obtained by electro-optical phase measurement techniques. Once the separation vector is re-established, after an SATPS signal interruption occurs, the phase integer combination for the two station is promptly redetermined, and static or kinematic surveying can continue.
Several benefits accrue from this total station approach: (1) rapid azimuthal angle determinations can be made; (2) use of differential SATPS information supplements and improves the accuracy of the survey parameters that can be measured; (3) SATPS signal processing can be done at the instrument station or at the mobile station; (4) where the frequency of the station-to-station data link is selected appropriately, or where one or more signal repeaters are used to relay signals between the two stations, survey measurements are not limited to line-of-sight measurements from instrument station to a mobile station, once the phase integer ambiguities are resolved; and (5) systematic and random errors in the SATPS and electro-optical measurements can be determined and reduced by combining the information from the two systems.